The present invention relates generally to optical fibers having graded index of refraction. More particularly, the invention relates to a method for forming an optical fiber having a graded index of refraction in the core and having high light transparency.
Step-indexed and gradient glass optical fibers typically have small diameters which render their connections to other parts of a communication system labor-intensive and cumbersome. In particular, connection of such glass fibers to another part of a system requires critical alignment of the fiber with a tolerance in the range of a few microns. A step-indexed fiber typically includes a core having a particular index of refraction surrounded by a cladding having a different index of refraction. Hence, a step-indexed fiber has a sharp discontinuity in the index of refraction at the boundary between the core and the cladding. Such step-indexed glass fibers generally have relatively low bandwidths, and hence are not suitable for many applications that require high rates of data transmission.
Further, plastics optical fibers including fibers having a gradient index of refraction with a particular shape are known. In contrast to step-indexed fibers, the index of refraction in fibers having a gradient index of refraction varies continuously in one direction, typically in a direction perpendicular to the longitudinal axis of the fiber, rather than having a sharp discontinuity. Such a refractive index gradient has typically a cylindrical symmetry and an approximately parabolic shape. Conventional plastic optical fibers are usually made of an amorphous polymer such as poly(methyl methacrylate) (PMMA). Such plastics fibers can be formed to have relatively large diameters, and can be employed for multi-mode transmission of data. These conventional plastics fibers, however, typically exhibit relatively large absorption coefficients in the infrared region of the electromagnetic spectrum, thereby limiting their applications in this region the spectrum. Further, such fibers can not be reliably employed at high temperatures and/or in humid environments.
U.S. Pat. No. 5,916,971 describes a graded refractive index fiber that is formed of an amorphous fluoropolymer and doped with a dopant material having a refractive index different from that of the fluoropolymer. The dopant material is said to be distributed within the amorphous fluoropolymer so as to produce the graded refractive index. The preferred method described in this patent for producing such a fiber includes producing a hollow tube formed of the fluoropolymer material and inserting a rod formed of the fluoropolymer material and the dopant material into the hollow tube. Heat fusing the rod with the tube produces the final distribution of refractive index across the tube. The insertion of the rod into the hollow tube requires producing the cavity in the tube with precise dimensions, thus adding to the cost and the time of the manufacturing process. Further, the distribution of the dopant material in the rod may not be uniform, and may not be the same for different rods, thus adversely affecting the reproducibility of the refractive-index gradient in the optical fiber and the quality of the gradient. In addition, there is a possibility of forming defects at the boundary between the rod and the tube upon heat fusion of the rod with the tube. Further, this process may not reliably secure the rod to the tube. That is, there is a possibility that the core splits from the cladding in the fiber.
Accordingly, it is an object of the present invention to provide a method for forming an optical fiber that is transparent over a wide range of the electromagnetic spectrum and is capable of efficient multi-modal transmission of optical data.
It is another object of the invention to provide a method for forming an optical fiber that can be employed for multi-modal transmission of digital optical data.
It is yet another object of the invention to provide a method that can reproducibly form an optical fiber having a graded refractive index.
The present invention provides a method of forming an optical fiber having a graded index of refraction in a plane perpendicular to its longitudinal axis. The term graded index of refraction as used herein refers to an index of refraction that varies continuously in a particular direction. The method the invention includes a step of providing a tube formed of an amorphous fluoropolymer having a first index of refraction. The tube has a longitudinal axis and a radial axis perpendicular to the longitudinal axis, and further includes a bore that extends along the longitudinal axis with an opening at an end thereof. The term xe2x80x9ctubexe2x80x9d as used herein can refer to a number of different structures which have a hollow interior portion open at one or both ends. The tube preferably has a circular cross section. Alternatively, the tube can be selected to have other shapes, such as parallelepiped or trapezoidal shape.
The method of the invention includes a step of filling the bore, formed in the tube with a liquid dopant material having a second index of refraction different from the first index of refraction to form a raw preform. The raw preform is heated for a sufficient duration to cause diffusion of the dopant material in the direction of the radial axis to form the desired graded refractive index. The heating of the raw preform further provides diffusion of the fluoropolymer into the filled bore to fuse the bore and form a finished preform having a core with the desired refractive index.
The finished preform may be drawn to produce an optical fiber having the graded index of refraction.
The bore is typically centrally located, and may have two openings at its opposed ends, each of which provides access to the space within the bore. The diameter of the bore can be selected to be a few millimeters, for example, one to two millimeters. If the bore has two openings, a plastic jacket can be placed around at least a portion of the tube, and clamped at one end to close one opening, thereby allowing filling of the bore with the liquid dopant material.
The amorphous fluoropolymer employed for forming the tube can be selected from a family of copolymers having selected ratios of polytetrafluroethylene (PTFE) and 2,2-bis(trifuoromethyl)-4,5-difluoro-1,3-dioxole. The dopant material is preferably an oligomer of trifluorochloroethylene.
When a copolymer of PTFE and 2,2-bis(trifuoromethyl)-4,5-difluoro-1,3-dioxol is employed to form the tube and triflurochloroethylene is employed as the liquid dopant material, the raw preform is heated to a temperature in a range of approximately 30xc2x0 C. above the glass transition of the copolymer, and is maintained at this temperature for a sufficient time, e.g., in the range of approximately 200 to 500 hours depending on the transport properties of the dopant and the desired configuration of the preform, such as core to cladding ratio, preform diameter and the numerical aperture. The heating of the preform causes diffusion of the liquid dopant in a radial direction to provide a graded distribution of the dopant material in the radial direction, and to fuse the core with the fluoropolymer tube. Because, the dopant material has an index of refraction that is different from the index of refraction of the fluoropolymer, the graded radial distribution of the dopant material leads to a graded refractive index in the finished preform. Those skilled in the art will appreciate that the method of the invention is not limited to fluoropolymers. In particular, any amorphous polymer/copolymer can be utilized in the method of the invention to form an optical fiber.
Another aspect of the invention provides a method for forming a transparent element having a graded refractive index. The method includes a first step of providing a tube of an amorphous fluoropolymer having a first index of refraction, and having a longitudinal axis and radial axis, and further having a bore extending along the longitudinal axis. The bore is formed to have at least one opening at one end thereof. Again, the bore is filled with a liquid dopant material having a second index of refraction which differs from the first index of refraction to form a raw preform. The raw preform is heated for a sufficient duration to provide diffusion of the dopant material in the direction of the radial axis to form the graded refractive index. The heating provides a sufficient flux of the fluoropolymer into the filled bore to fuse the filled bore, thereby forming the transparent element with a core having the graded refractive index.
A further aspect of the present invention provides an optical fiber by first extruding an amorphous fluoropolymer to form a preform having a longitudinal axis and a radial axis, and further having a core extending along the longitudinal axis. The core has at least an opening that allows introducing a liquid dopant material into the volume subtended by the core, to form a raw preform. The dopant material is selected to have an index of refraction that is different from the index of refraction of the fluoropolymer. After introduction of the dopant into the core, the raw preform is heated for a sufficient duration to provide diffusion of the dopant material in the direction of the radial axis to form the graded refractive index. The heating of the preform further provides a sufficient flux of the fluoropolymer into the filled bore to fuse the filled bore, thereby forming a finished preform with a core having the graded refractive index. In a final step, the finished preform is drawn to form an optical fiber having the graded index of refraction.
In still another aspect, the present invention provides an optical fiber and/or a transparent element produced by any of the above recited methods.